Intensity-coupled Polarization in Instruments with a Continuously Rotating Half-wave Plate
2019
We discuss a systematic effect associated with measuring polarization with a continuously rotating half-wave plate
(HWP). The effect was identified with the data from the E and B Experiment, which was a balloon-borne
instrument designed to measure the polarization of the cosmic microwave background (CMB) as well as that from
Galactic dust. The data show polarization fractions larger than 10%, while less than 3% were expected from
instrumental polarization. We give evidence that the excess polarization is due to detector nonlinearity in the
presence of a continuously rotating HWP. The nonlinearity couples intensity signals to polarization. We develop a
map-based method to remove the excess polarization. Applying this method to the 150 (250) GHz band data, we
find that 81% (92%) of the excess polarization was removed. Characterization and mitigation of this effect are
important for future experiments aiming to measure the CMB B-modes with a continuously rotating HWP
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